[petsc-dev] coarse grid in gamg (for the OPTE paper)

[Barry Smith]

On Sep 19, 2013, at 4:28 PM, "Mark F. Adams" <mfadams at lbl.gov> wrote:

> 
> On Sep 19, 2013, at 3:53 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
>> 
>>  It is essentially 3 Laplacians
> 
> If you mean literally 3 scalar laplacians packed in one matrix for some reason then the one constant vector is fine.  

  Really, but (for periodic or N. bc) the null space has three vectors, you don't need all of them in your near null space?


> Is this a block diagonal matrix with 3 big blocks, if you order it correctly of course?
> 
>> so I think the default null space of 3 constant vectors is fine.
> 
> You get this if you set the block size to 3.  Constant vectors in each of the three components.
> 
>> The problem is without the block information presumably GAMG is using only a single constant vector over all variables?  So maybe we need to construct a 3 vector null space which just marks in the reduced vector from which of the 3 components each entry came from.
>> 
>>  Barry
>> 
>> On Sep 19, 2013, at 3:48 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>> 
>>> Barry Smith <bsmith at mcs.anl.gov> writes:
>>>> That is what it is doing and apparently it doesn't result in a good
>>>> preconditioner; I don't know why off hand. One thing is it no
>>>> longer knows about the block structure.
>>> 
>>> How is the near null space being specified?
>> 
>